Page 206 - Read Online

Page 18 of 30 Guo et al. Microstructures 2023;3:2023038 https://dx.doi.org/10.20517/microstructures.2023.30

Competition of chlorine oxidation in OER processes
Taking into the total concentration of Cl in seawater is approximately 0.54 M, Yu et al. plotted a simulated
-
Pourbaix diagram of oxygen/chlorine electrochemistry in seawater to evaluate the relationship between the
oxygen electrocatalytic reaction and the ClER during cycling, as shown in Figure 11 . During charging, Cl
[53]
-
in seawater may participate in the HCFR or ClER process, generating hypochlorite or chlorine byproducts.
Among all pH ranges, OER is thermodynamically preferred over HCFR and ClER. Particularly, OER
electrocatalysts functioning at an overpotential of 480 mV in 0.1 M KOH solution can block the HCFR
process in seawater electrolytes, although this is difficult at relatively large current densities. Moreover, the
potential difference between oxygen and chlorine becomes slightly less with a reduction of the pH value
(< 7.4), where HClO replaces ClO as the main production. When the pH value is less than 2.9, the
-
equilibrium potential is near to but still higher than the 100 ~ 200 mV required by OER criteria. Thus, the
OER process has higher selectivity and feasibility in the electrolyte with the presence of Cl .
-

The OER process would compete with the HCFR/ClER process in terms of reaction kinetics even though
the OER process is thermodynamically superior to the HCFR/ClER during the charging process of
-
SMABs . This is because HCFR/ClER is a 2e oxidation reaction involving only a single intermediate,
[109]
whereas OER is based on a complex 4e pathway that requires the removal of four protons and involves
-
three intermediates. Moreover, the two reaction routes would partially share similar active sites. By the
theory calculations based on assumed mechanisms, Hansen et al. discovered a scaling relationship between
[110]
the binding energies of ClER and OER intermediates . This indicates that electrocatalysts that easily bind
oxygen-bound intermediates also tend to bind chloride-bound intermediates. The pH values, current
density, and Cl content would also have significant impacts on this competition. Since HCFR cannot occur
-
when the electrocatalysts provide a low overpotential, alkaline conditions are consequently preferable for
selective OER in saltwater.

OER electrocatalysts in chlorine-containing electrolyte.
For the R-SMAB, the charging process depends on the OER catalysis on the air electrode. The four-electron
reaction steps for the OER process would result in sluggish reaction kinetics, leading to large overpotentials;
thus, the high charging voltage would restrain the performance of metal-air batteries in practice [108,111] . In the
complex seawater-based electrolyte, the main challenge is the HCFR process, competing with OER under
near-neutral or alkaline conditions, together with the consequent electrocatalyst corrosion [112,113] . Over the
past decades, many OER electrocatalysts have been designed and prepared with excellent activity, selectivity,
and stability in the chlorine-containing electrolyte, mainly including metal hydroxide-based electrocatalysts,
TM compound-based electrocatalysts and hybrids, or compositing electrocatalysts. Furthermore, since the
anode during seawater electrolysis also depends on the OER reaction, in order to comprehensively
understand the impact of Cl on the OER reaction in seawater and the synthesis strategies for chloride-
-
resistant OER electrocatalysts, this chapter will discuss the studies related to seawater electrolysis together.

Metal hydroxide-based electrocatalysts
Initiatively, Dionigi et al. established a general designing criterion for noble metal-free OER electrocatalysts,
pointing out that the OER overpotential keeping smaller than 480 mV could achieve high selectivity in
[114]
seawater in theory . The designing criterion is the only one and the most favorable condition to avoid
HCFR from the air electrodes of seawater-based metal-air batteries. As shown in Figure 12A, a NiFe-layered
double hydroxide (NiFe-LDH) catalyst was prepared, and the linear sweep voltammetry (LSV) curves of
OER were measured in 0.1 M KOH and 0.3 M borate buffer solution with/without the addition of
0.5 M NaCl. The OER process was more likely to occur at pH = 13 than at pH = 9.2, and the overpotential
-2
-2
increased by 110 mV at 1 mA cm . Furthermore, the overpotential at 10 mA cm in pH = 13 reached
360 mV, which was not over 480 mV and satisfied the above criterion. Therefore, there were no changes for